Electrically conductive organic polymers have become of scientific and technical interest since the late 1970s. The polymers, which are based on a comparatively new technique, exhibit electronic and magnetic characteristics of metal as well as physical and mechanical characteristics of conventional organic polymers. Known conductive organic polymers include poly(p-phenylene)s, poly(p-phenylenevinylene)s, polyanilines, polythiophenes, polypyrroles, polyazines, polyfurans, polycenophenes, poly(p-phenylene sulfide)s, mixtures thereof, blends thereof with another polymer, and copolymers of monomers of the above-described polymers. These conductive organic polymers are conjugated-system polymers which exhibit electrical conductivity through doping caused by reaction such as oxidation, reduction, or protonization.
In recent years, efforts have been made to fabricate, from these conductive organic polymers, light-emitting elements of organic electroluminescent devices (organic EL, OLED) and active elements of field-effect transistors (organic FET, organic TFT). In one current practice, an expensive plasma CVD apparatus is used for forming an insulating layer or a semiconductor layer of an amorphous silicon TFT or polysilicon TFT, and an expensive sputtering apparatus is used for forming an electrode. In addition, film formation by CVD must be carried out at a temperature as high as 230 to 350° C., and maintenance operations such as cleaning must be carried out frequently, thereby reducing throughput. In contrast, apparatuses such as a coating apparatus and an ink-jet apparatus for fabricating organic FETs or similar devices are less expensive than the CVD apparatus and sputtering apparatus. In addition, film formation can be performed at lower temperature, and maintenance of the apparatuses is less cumbersome. Therefore, when display devices such as a liquid crystal display and an organic EL are fabricated from an organic FET, a remarkable cost reduction can be expected.
Typical organic EL devices include a transparent substrate made of material such as glass, a transparent electrode, a hole-injecting layer, a hole-transporting layer, a light-emitting layer, an electron-transporting layer, and a metal electrode. Three separate layers; namely, the hole-transporting layer, the light-emitting layer, and the electron-transporting layer, may be formed into a single hole-transporting and light-emitting layer, or into a single electron-transporting and light-emitting layer. The specific features are disclosed by Japanese Patent Application Laid-Open (kokai) Nos. 7-126616, 8-18125, 10-92576, etc. However, problems such as service life still remain unsolved for organic EL devices, and studies for improvement are under way.
Typical organic TFTs include a transparent substrate made of material such as glass, a gate substrate, a gate insulating film, a source electrode, a drain electrode, and an organic semiconductor film. By modifying gate voltage, electric charge at the interface between the gate insulating layer and the organic semiconductor film is rendered excessive or deficient, whereby the drain current flowing between the source and drain electrodes via the organic semiconductor film is varied, to thereby perform switching.
Japanese Patent Application Laid-Open (kokai) No. 63-076378 discloses that an organic TFT is fabricated from polythiophene or a polythiophene derivative serving as the aforementioned organic semiconductor film. Fabrication of an organic TFT from pentacene is disclosed in Yen-Yi Lin, David J. Gundlach, Shelby F. Nelson, and Tomas N. Jackson, IEEE Transaction on Electron Device, Vol. 44, No. 8, p. 1,325 (1997).
However, use of pentacene raises problems. For example, film formation must be performed through a vapor deposition process, and crystallinity must be elevated for enhancement of device characteristics. Another possible approach is use of a soluble pentacene derivative for enhancing processability. However, in this case, characteristics remain unsatisfactory.
Application and development of an organic semiconductor formed of polythiophene, a polythiophene derivative, or a thiophene oligomer are under way, since the organic semiconductor has excellent formability; e.g., is readily formed into thin film through electrolytic polymerization, solution coating, or a similar method. However, in this case, characteristics remain unsatisfactory.
Meanwhile, in recent years, hyperbranched polymer materials in a broad sense such as dendrimers and hyperbranched polymers have become of interest. Characteristic features of dendrimers and hyperbranched polymers include amorphousness, solubility in organic solvent, and presence of a large number of branch ends to which a functional group can be introduced. L. L. Miller et al. describe in J. Am. Chem. Soc. 1997, 119, 1,005 that a polyamide dendrimer having, at branch ends, 1,4,5,8-naphthalenetetracarboxy-diimido residues to which a quaternary pyridinium salt is bonded has isotropic electron conductivity (also referred to as “transportability”), and that the conductivity is provided by interaction of π electrons generated by spatial overlapping of the branch end moieties. Japanese Patent Application Laid-Open (kokai) No. 2000-336171 discloses a dendrimer containing a dendron having hole-conducting moieties at branch ends and no π-electron-conjugated system including a carbonyl group and a benzene ring, as well as a photoelectric conversion device employing the dendrimer.
However, in function elements employing semiconductive or conductive polymers such as conjugated polymers, high charge conductivity of the aforementioned organic semiconductor appears along a molecular chain orientation, and varies depending on the molecular structure. In addition, semiconductive or conductive polymers such as conjugated polymers are generally rigid and cannot be dissolved or melted. Most of them cannot be dissolved in solvent. To this end, there are used derivatives of such polymers into which side chains are introduced, and oligomers thereof (see Japanese Patent Application Laid-Open (kokai) Nos. 4-133351, 63-076378, 5-110069, etc.). However, problems also arise. For example, when side chains are introduced, glass transition temperature appears, and thermochromism attributed to micro-Brownian motion is induced, resulting in temperature-dependent variation in characteristics. Use of oligomers may deteriorate reliability. Even when the side-chain-introduced polymer is used, satisfactory mobility cannot be attained. Thus, polymerization degree must be increased, or orientation degree of the conductive organic compound must be enhanced by use of orientation film as described in, for example, Japanese Patent Application Laid-Open (kokai) No. 7-206599.
Furthermore, since conjugated polymers tend to be affected by oxygen and water, thereby readily causing deterioration, conventional organic FET elements employing the conjugated polymers have poor stability and electric characteristics and a short service life, which is problematic.
Meanwhile, a hyperbranched polymer having a thienylene-phenylene structure as a structural repeating unit is disclosed in literature (Japanese Patent No. 3074277B). However, the disclosed compounds are used as conducting materials after being doped with an electron-accepting reagent to thereby form conductivity-imparted polymer. Since the production method disclosed in this document employs polymerization based on the Grignard reaction, such a highly regulated repeating structure as a dendrimer has cannot be obtained. Thus, the compound synthesized through this method has a wide molecular weight distribution profile, as is the case with customary polymers. In addition, when functional groups are attempted to be introduced to a core serving as a center moiety or end groups serving as a molecular surface, these functional groups are introduced randomly, raising a problem that a desired function is difficult to attain.